Currently widely used display devices in which liquid crystal compounds are used are usually driven by TN (twisted nematic) mode.
When driving by TN mode is adopted, however, the positions of liquid crystal compound molecules in the element of the device must be changed in order to change a displayed image. As a result, there are involved problems that the driving time of the device becomes prolonged, and the voltage required for changing the liquid crystal compound molecular position, namely, power consumption becomes large.
Switching elements incorporating ferroelectric liquid crystal compounds, different from those in which TN mode or STN mode is utilized, can be functioned only by changing the molecular orientation direction of the liquid crystal compounds, and hence the switching time is markedly shortened. Further, the value Ps x E obtained from a spontaneous polarization (Ps) of the ferroelectric liquid crystal compound and an intensity of the electric field (E) applied is an effective energy output for changing the molecular orientation direction of the liquid crystal compounds, and accordingly the power consumption is also significantly diminished. Such ferroelectric liquid crystal compounds as mentioned above have two stable states, namely, bistability, in accordance with the direction of the applied electric field, and therefore show significantly excellent switching threshold value characteristics. Accordingly, the ferroelectric liquid crystal compounds are particularly appropriate for display devices for animations.
When these ferroelectric liquid crystal compounds are used in optical switching elements, etc., they are required to have various characteristics such as an operating temperature in the vicinity of or not higher than room temperature, a wide operating temperature range, a high switching speed (quick), and a switching threshold value voltage value in an appropriate range. Of these characteristics, the operating temperature range is a particularly important property when the ferroelectric liquid crystal compounds are put into practical use.
So far as ferroelectric liquid crystal compounds known hitherto are concerned, however, they have drawbacks such as a generally narrow operating temperature range, and an operating temperature range in the high temperature region not including room temperature even when their operating temperature range is wide, as disclosed, for example, in R. B. Meyer et al., J. de Phys., Vol. 36 L, p 69 (1975) and a paper reported by Masaaki Taguchi and Takamasa Harada, "Proceedings of Eleventh Conference on Liquid Crystal" p 168 (1985). Thus, there are no available ferroelectric liquid crystal compounds that are satisfactory from the standpoint of practical use.